Presensitized (PS) plates comprising a positive type sensitizing agent containing a diazo compound and a phenolic resin as main components or a negative type sensitizing agent containing an acrylic monomer or prepolymer as a main component have heretofore been used practically as lithographic offset printing plate precursors. However, since all of these printing plate precursors have a low sensitivity, these printing plate precursors are exposed to light with a film original, on which an image has been previously recorded, brought into close contact therewith to form printing plates. On the other hand, with the advances in computer image processing techniques and large capacity data storage and communication techniques, a continuous computer operation including original input, correction, editing, layout and paging has been enabled. With this computer operation, an electronic editing system capable of instantly outputting data to terminal plotters via a high speed communications network or a satellite communications network has been used practically. In particular, such an electronic editing system is in great demand in the field of instantaneous newspaper printing. Furthermore, in the field of printing wherein a printing plate is reproduced as required based on an original stored in the form of a film original, there is a growing tendency to store originals as digital data in very large capacity recording media such as optical discs.
However, little or no direct type printing plate precursors designed to receive data directly from the output of terminal plotters to form a printing plate have been put into practical use. Even in stations where an electronic editing system is operated, data is outputted to a silver salt system photographic film. PS plates are then exposed to light with the silver salt system photographic film brought into contact therewith to form printing plates. One of the reasons for the above described situation is that it is difficult to provide a direct type printing plate precursor having sufficient sensitivity to form a printing plate within a practical period of time using a light source in the output plotter (e.g., a He-Ne laser, a semiconductor laser).
An electrophotographic photoreceptor is a light-sensitive material with a light sensitivity high enough to provide a direct type printing plate. Many electrophotographic printing plate precursors of the type wherein a photoconductive layer in the nonimage portion is removed after the formation of toner image are already known. Examples of such electrophotographic printing plate precursors include those described in JP-B-37-17162, JP-B-38-6961, JP-B-38-7758, JP-B-41-2426 and JP-B-46-39405 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and JP-A-50-19509, JP-A-50-19510, JP-A-52-2437, JP-A-54-145538, JP-A-54-134632, JP-A-55-105254, JP-A-55-153948, JP-A-55-161250, JP-A-57-147656 and JP-A-57-161863 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In order to prepare a lithographic printing plate (offset printing plate) using such an electrophotographic lithographic printing plate precursor, the electrophotographic lithographic printing plate precursor which has been imagewise exposed to light is first subjected to a toner development process to form a toner image. These toner development processes have include a known positive development process which comprises development of portions on which electrostatic latent images remain and a reversal development process which comprises development of portions on which electrostatic latent images do not remain. In general, where a light source such as a laser is used to achieve a scanning exposure, a reversal development process is often used. Thereafter, the toner image is fixed. The nonimage portions, other than the toner image portion, are then eluted with an elute so that the hydrophilic substrate is exposed to prepare a lithographic printing plate.
Printing from such a lithographic printing plate gives rise to certain difficulties. When the edge of the printing plate does not come into contact with the surface to be printed, e.g., as in ordinary sheet printers wherein paper sheets with a smaller size than the printing plate are printed, no difficulties occur. However, when a rotary press as used in printing newspapers to print on paper rolls, linear stain are produced on portions of the printed matter corresponding to the edge of the printing plate. This stain occurs markedly when a reversal toner development process is used.
Techniques for inhibiting stain due to the edge of a lithographic printing plate obtained by reversal development of an electrophotographic lithographic printing plate precursor include use of an insulating resin layer on the edge face (side) of the electrophotographic lithographic printing plate precursor (JP-A-63-178240). This approach is based on the idea that print stain due to the lithographic printing plate obtained by reversal development of an electrophotographic lithographic printing plate precursor occurs by toner attaching to the edge face of the electrophotographic lithographic printing plate precursor upon reversal development which leads to the attachment of ink thereto upon printing. Thus, this phenomenon can therefore be inhibited by coating an insulating resin on the edge face of the electrophotographic lithographic printing plate precursor. If such an insulating resin layer is provided on the edge face of the printing plate precursor, little or no toner is attached thereto. However, it was found that since the insulating resin remains on the edge face of the printing plate, ink attaches thereto, causing print stain. In particular, the attachment of the toner can be inhibited by providing an insulating resin layer on the edge face of the printing plate precursor. However, it was found that since the insulating resin layer thus provided has a lipophilic nature itself (i.e., ink receptivity), its presence causes the ink to attach to the edge face of the printing plate precursor, making it impossible to inhibit print stain.
JP-A-2-61654 and JP-A-2-66566 disclose providing a water-soluble high molecular weight layer or a resin layer having a higher solubility in an alkaline solution than the light-sensitive layer on the edge face of the printing plate precursor. These approaches are intended to remove the attached toner together with the resin layer by improving the solubility of the resin layer in an etching solution. However, these approaches are disadvantageous because the resin is coated on the edge face of the printing plate precursor in the form of solution, the coating solution extends to portions other than the edge face, e.g., light-sensitive layer or back surface, inhibiting the formation of images on the light-sensitive layer. It was further found that once toner is attached to and fixed on the portion of the resin layer which had extended to the light-sensitive layer, the toner cannot be removed even using an etching solution. The reason for this phenomenon is unknown. It is postulated that the components in the light-sensitive layer, the resin for edge face treatment and the toner interact in some manner, making the toner insoluble in the etching solution. Thus, the above described approach cannot solve the problem of stain on the edge portion unless it is possible to coat the resin only at the edge portion in the form of solution. Further, if the resin is coated on the edge face of a stack of sheets of printing plates, the coating solution which penetrates into the gap between is the sheets dries and glues the printing plates together, and the printing plates can not be freely used.
Further, JP-A-1-261660 discloses physical removal of a toner attached to the edge face of an electrophotographic printing plate precursor. However, this approach is disadvantageous because the toner is not completely removed and an additional apparatus which adds to the cost of the process is required.
No edge face treatments have been found for completely removing toner attached to the edge face of an electrophotographic lithographic printing plate precursor in an etching solution to inhibit stain on the edge face of a lithographic printing plate produced by a reversal development of an electrophotographic lithographic printing plate precursor. Nor has any means been found for definitely coating such an edge face treatment only on the edge face of the printing plate.